Colloidal dispersions such as emulsions are dispersed systems consisting of two or more mutually insoluble or sparingly soluble liquids. One of the liquids is usually present in excess and is termed the continuous or external phase, while the liquid dispersed in it is termed the dispersed, discontinuous or internal phase. If the continuous phase consists of water, and the dispersed phase consists of an organic liquid, such as mineral oil, the term oil-in-water (O/W) emulsion is used. If water is finely dispersed in an organic or non-aqueous liquid, a water-in-oil (W/O) emulsion is produced. If two organic liquids are emulsified in each other, the term oil-in-oil (O/O) emulsion or dispersion is used.
While O/W and W/O emulsions containing a non-polar oil such as silicone are common, O/O emulsions in which both phases are essentially non-polar are relatively rare. The Journal of Colloid and Interface Science, Volume 195, Pages 101-113, Article No. CS975158, Jan. 1, 1997, describes certain paraffin oil-in-silicone oil O/O emulsions, as well as certain silicone oil-in-paraffin oil O/O emulsions. Similarly, emulsions of castor oil in silicone oil, as formulations for drug delivery, is described in the Journal of Drug Deliver Science and Technology (2004), 14(2), 113-117.
US Patent Publication No. 2004/0002429 describes lubricant compositions comprising an emulsion comprising a low viscosity, relatively non-polar, hydrocarbon carrier fluid and a minor amount of an immiscible or semi-miscible polar, hydrocarbon fluid.
PCT Appl. WO2003/000396 A1 describes emulsions comprising silicones, as either the continuous phase or the dispersed phase, stabilized by graft and block copolymers, useful for cosmetic applications.
U.S. Pat. No. 6,080,394 A discloses a non-aqueous polar solvent-in-oil emulsion composition containing a non-aqueous polar solvent phase dispersed in a silicone oil continuous phase by an emulsifier. U.S. Pat. No. 6,238,657 B1 describes stabilized O/O emulsions where one of the oil phases is silicone oil while the other oil phase is an organic oil such as mineral oil or castor oil, as well as three-phase aqueous emulsions derived from these and the use of such multi emulsions in personal health care applications.
The formation of O/O emulsions in aliphatic hydrocarbons or the like, such as dodecane, having low-dielectric constants is not trivial. In general, in the formation of emulsions, stable dispersion of droplets or particles result when the attractive potential between two droplets is less than repulsive potential. As repulsive potential is directly proportional to the dielectric constant of the dispersion medium, stable dispersions cannot be easily achieved in a medium of very low dielectric constant such as aliphatic hydrocarbons.
Another issue with which to contend, in the case of particles dispersed in low density hydrocarbon solvents such as dodecane, is settling of the dispersed phase with time, as governed by Stokes law that defines settling velocities of particles in a fluid by the following equation:V=[(2gr2)(d1−d2)]/9μwhere V=velocity of settling, g=acceleration due to gravity, r=radius of particle or dispersed phase, d1=density of dispersed phase, d2=density of medium, and μ=viscosity of the continuous phase. The issue of settling or creaming of particles is especially relevant to electro-optical modulating display devices in which particles are dispersed in a liquid system, such as electrophoretic, electrowetting, or electrochromic display devices. It is important that the particles in such systems remain neutrally buoyant, neither creaming nor settling. Since viscosity and density mismatches of the dispersed phase, typically solid particles, and the continuous phase are usually so large, techniques such as increasing the viscosity of the continuous phase using polymeric additives are employed to overcome this effect. Such solutions, however, can result is potential drawbacks, for example, causing the electrical mobility of the particles to be compromised.
Given the difficulty and rarity of obtaining oil-in-oil emulsions in general, obtaining such an emulsion in which the two phases have certain desirable properties, which may be advantageous for a given application, is especially challenging. For example, using silicone oil as the dispersed phase can limit the additives that can be solubilized or dispersed effectively in them for many particular applications.
Therefore, there is a need for O/O emulsions in which both the continuous phase and the dispersed phase can be designed to have certain desirable properties, or combinations of properties, which cannot be obtained with prior-art O/O emulsions such as those in which silicone oil is one of the phases. Among the properties that may be desired for both or one of the emulsion phases, depending on the application, include non-polarity as evidenced by a low-dielectric constant. In addition, it may be desired that the dispersed phase is capable of readily incorporating into the dispersed phase, colorants, polymers, or other additives. O/O emulsions having improved or advantageous properties would be useful for a variety of applications involving oil-in-oil emulsions in general. In addition, O/O emulsions with properties not heretofore obtainable, would offer the opportunity for the development of new applications for such materials.